Synchronised magnetic escapement



Sept. 15, 1964 c. F. CLIFFORD ETAL 3,148,497

sYNcHRoNIsED MAGNETIC ESCAPEMENT Filed May 23, 1962 @a i 'lr-@f4 UnitedStates Patent O "ice 3,148,497

3,143,497 SYNCHRGNESED lt/EAGNE'IEC ESCAPEMEN'I Cecil F. Ciifford andJonathan Anthony El. Key, both of Newhridge Works, Enth, Somerset,England Filed May 23, i962, Ser. No. 197,823 Claims priority,application Great Britain .lune 1, 1961 Claims. (Cl. 58--26) Thisinvention relates to mechanical or electromechanical oscillators of thekind used for controlling the operation of clocks, and more particularlyto a magnetic escapement in which the oscillating member is a magnet.

A magnetic escapement is known in which an escapement wheel made ofmagnetic material is formed with teeth around its periphery, andopenings delining spokes, in such a manner as to leave a circular wavytrack aro-und the wheel. This wheel co-operates with a magnet mounted tooscillate in a direction substantially radial to the wheel .axis andhaving its two poles presented to opposite faces of the wavy track onthe wheel, so that as the magnetic system oscillates the wheel isallowed to rotate at a speed determined by the natural frequency ofoscillation of the magnetic system. The present invention is directed toan improvement in such a system which is applicable to a clock or othertiming mechanism adapted to be synchronised with the supply mainsfrequency.

The invention is particularly applicable to a known type of clock ortiming mechanism or the like which is normally driven from alternatingcurrent :supply mains but `also contains a spring which keeps the clockoperating for a substantial period if the supply mains should fail. Itwill, however, be realised that the invention or variations thereofwithin its scope may be applied to other types of clock or timingmechanism.

In a clock of the type above referred to, a spring is provided, withappropriate gearing, to drive a wheel having teeth formed around itsperiphery and having apertures cut inside the teeth to form spokes, thespokes being in staggered relationship .to the teeth in such a mannerthat a solid ring of material is left around the wheel which due to thematerial removed in forming the teeth and spokes constitutes the wavytrack.

In normal operation .the spring is kept fully wound by a motor driven bythe electric mains, to ensure that the clock operates correctly eitheras a synchronous or a spring driven clock. The natural frequency of themagnetic system is adjusted to correspond as' closely as possible to themains frequency and electromagnetic means are provided to act upon themagnetic system for synchronising with the mains frequency. Suchelectromagnetic means are in themselves known and may comprise a windinghaving a magnetic core which is' closed except for a gap and a smallarmature of magnetic material attached to the oscillating magneticsystem, the electromagnet being energised at the mains frequency.

In normal operation the escapement wheel is driven by the spring, whichis kept fully wound by the motor driven by the electric mains. Theescapement wheel rotates and sets the magnetic system in oscillation,the speed of rotation of the wheel being controlled by the frequency ofoscillation of the magnetic system. The pulses applied by theelectromagnet cause the magnetic system to vibrate synchronously with.the mains frequency so that the clock runs as a synchronous clock. Incase of failure of the supply mains the spring continues to drive theescapement wheel and the clock continues to run until the spring is rundown, unless the mains supply is restored in the meantime. There may bea small time error due to the fact that the natural frequency of themagnetic system may not correspond exactly with the nominal frequency ofthe supply mains and, furthermore, there may be a slight temperatureerror. These errors are, of course, kept to a minimum by appropriatedesign. As soon as the mains supply is restored, the electromagnet againtakes control and the magnetic system is made to oscillate at thesynchronous frequency, while the motor rewinds the spring.

A ditliculty experienced with this type of clock is that if the naturalfrequency of the magnetic system corresponds exactly with that of themains frequency the application of the synchronising impulses throughthe electromagnet produces a resonance effect, so that the amplitude ofoscillation of the magnetic system increases until the systemo-scillates with too great an amplitude. This may lead to inconsistentand noisy running or even cause the oscillating member to strike otherparts. To reduce this tendency it is necessary to keep the powersupplied by the eleotromagnet is to a very small level. if the mainsfrequency should depart to more than a small extent from the nominalfrequency then it will no longer correspond with the natural frequencyof the oscillating system and the amount of power supplied to theelectromagnet may be insufficient to maintain reliable synchronisation.The supply frequency, nominally 50 c./S., may vary between about 48 and50.5 c./s.

The invention consists of a ytime control mechanism comprising anescapement wheel having a wavy magnetic track therearound and anoscillatory magnetic system cooperating therewith, an armatureconsisting of a at thin strip of magnetic material attached to themagnetic system so as to be oscillatory therewith about a reposeposition, and an electro-magnet having a winding and a magnetic circuitas a thin strip of magnetic material with two ends thereof presentingopposed co-planar pole pieces separated to provide a gap in the magneticcircuit, the armature being oscillatory in a path which extends at leastinto the gap, and the plane of the armature being substantially parallelto the common plane of the coplanar pole pieces when the armature is inthe repose position.

In one form of the invention the armature in its repose position may liein the same plane as the magnetic circuit.

In another form the plane of the ar-mature in its repose position is sofar removed from the plane of the magnetic circuit that the armature atits maximum amplitude of oscillation in one direction just cornes intoalignment with magnetic circuit.

In another form the plane of the armature in its repose position isspaced to a lesser degree from the plane of the magnetic circuit core sothat in its oscillation in the one direction the armature moves into andpast the position of alignment with the electromagnetic core.

In a modification of any of the above-described ernbodiments a half-waverectifier may be incorporated in the supply circuit to the electromagnetwinding so that only half-waves of current are supplied thereto, wherebythe frequency of oscillation of the magnetic system may be halved.

To promote a ready understanding of the invention certain embodimentsthereof will now be described, by way of example, with reference to theaccompanying drawings in which- FIGURE l shows in diagrammatic pictorialform one arrangement of the invention;

FIGURE 2 is a diagram showing the armature arranged so that in itsrepose position it is in alignment with, i.e. in the same plane as, themagnetic circuit;

FIGURE 3 is a diagram showing the repose position of the armature in aplane displaced by one-half the total oscillation amplitude from theplane of the magnetic circuit;

FIGURE 4 is a diagram showing the repose posltion of the armature in aplane displaced from the plane of the magnetic circuit by an amountequal to one-quarter of its total oscillation amplitude;

FIGURE 5 is a diagram showing the armature in its repose position in aplane displaced from the plane of the magnetic circuit by an amountintermediate between the amounts shown in FIGURES 2 and 4; and

FIGURE 6 is a diagram showing the winding of the electromagnet having arectifier incorporated in its supply circuit, whereby the oscillationfrequency of the magnetic system may be halved.

Referring initially to FIGURE 1, a known type of magnetic escapement isshown in which an escape wheel I1 is supported on a spindle 12 which iscarried in bearings (not shown) so that the escape wheel may rotatefreely. The escape wheel Il is made of magnetic material, for example,magnetic iron having low hysteresis, and is provided with teeth I3. Inradial alignment with each tooth is a hole 14 so that the contour of theteeth and holes constitute a wavy track of magnetic material around thewneel. A U-shaped permanent magnet 15 has its open ends I6 and I7 turnedinwardly towards each other to leave a gap which is somewhat larger thanthe thickness of the escape wheel Il. The magnet I5 lies in a planewhich is approximately tangential to the average diameter of the wavytrack formed by the teeth and the holes in the wheel Il.

The magnet I5, at the closed end of the U, is attached to one end of areed IS, which may conveniently consist of a ribbon of springy materialhaving a substantially zero thermo-elastic co-ecient, and the reed 13lies within the U formed by the magnet I5. The other end of the reed I8is attached by means of a screw I9 to a bracket Ztl which represents aportion of a fixed structure or framework. The parts are preferablyarranged so that the magnet may oscillate about an axis containing thecentre of gravity of the magnet and reed assembly. Since the magnet ispositioned so that its ends I6 and 17 are substantially tangential tothe average diameter or pitch circle of the wavy track on the escapewheel lll, oscillation of the magnet will control the speed of rotationof the wheel Il, which is normally driven by a spring.

In experiments with a system of this type it has been found that if thearmature attached to the magnetic system is made of a fairly thin stripof magnetic material of low hysteresis and, for example 0.5 millimetrethick, as shown at 2l in FIGURE l, and the system is so arranged thatthis strip oscillates in a gap 22 in a magnetic circuit 23 also made ofa strip of magnetic material having an equal thickness, the core beingprovided with a winding 24, then Various effects can be producedaccording to the repose position of the armature with respect to thepoles of the electromagnet. These will now be described.

FIGURE 2 is a diagram showing the armature ZI, in its repose positionlying in alignment with, i.e., in the same plane as, the two ends of themagnetic circuit 23, so that when oscillating it moves by a distance Ain each direction away from its repose position to a position 2in. Thetotal oscillation amplitude is thus 2A. Under this condition theapplication of current to the electromagnet exerts an additionalcentralising force on the armature and thereby effectively stitfens thesystem, so that the natural frequency of oscillation of the oscillatingmagnetic system is raised to a frequency substantially above theoriginal natural frequency of the mains. In a given system, the amountby which the natural frequency of the magnetic system is raised dependsupon the strength of the electromagnet, and therefore upon themagnetizing ampere turns. If such a system is to be used with a 50 c./s.mains supply then the natural frequency of the oscillating magneticsystem is made as closely as possible 50 c./s. When the winding 24 isenergized thernatural frequency of the magnetic system is raised wellabove 50 c./s. but the electromagnet forces the magnetic system tocontinue to oscillate at 50 c./s. Since the magnetic system is no longeroscillating at its natural frequency, resonance effects are avoided andan appreciable amount of power must be applied to the winding 24 toforce the magnetic system to continue to oscillate at a frequency of 50c./s., so that reliable oscillation is assured.

FIGURE 3 is a diagram similar to FIGURE 2, but in this case the reposeposition of the armature 21 has been displaced so that the armature isno longer in alignment with the ends of the magnetic circuit 23, but themagnetic circuit and the armature lie in parallel planes, thedisplacement D being equal to the amount by which the armature movesfrom its repose position during oscillation; thus the dimension D isequal to the half-amplitude dimension A and the armature 2li comes intoalignment with the magnetic circuit 23 during one half-cycle of itsoscillation and during the succeeding half-cycle it is moving away fromthe core 23. Under this condition the natural frequency of theoscillating system is reduced, the degree of reduction again beingdependent upon the magnetising ampere turns of the winding Z4.

With this arrangement there is a pull on the armature 2li tending topull it into alignment with the magnetic circuit 23 at each half-cycleof alternating current. That is to say, the system oscillates at doublethe mains frequency. Thus the oscillating magnetic system would beconstructed so that its natural frequency of oscillation is 1G() c./s.but when the winding 24 is energized the natural frequency of theoscillating system would be reduced to a frequency below c./s. and itwould be forced to oscillate at a frequency of 1GO c,/s. due to theelectromagnet.

FIGURE 4 shows an arrangement in which the plane of the armature in itsrepose position is displaced from the plane of the magnetic circuit byabout half the distance it moves from its repose position duringoscillation, that is to say, the displacement D is equal to onehalf A,so that during one half-cycle of oscillation the armature ZI moves awayfrom the magnetic circuit and during the other half-cycle it moves intoand past the position of alignment to the extent of about half itsmaximum displacement from the neutral point. The maximum synchronisingeffect is produced with this arrangement for a given amount of powerapplied to the winding 24, and the frequency of oscillation of themagnetic system is double the mains frequency. Thus for operation on 50c./s. mains the oscillating magnetic system will be constructed to havea natural frequency of 100 c./s. and energisation of the coil 24 willcause the natural frequency of the oscillating system to be shifted awayfrom the 100 c./s. frequency, but the electromagnet will force theoscillating system to oscillate at a frequency of 100 c./s.

FIGURE 5 is a diagram of an arragnement which is a compromise betweenthe three conditions described in relation to FIGURES 2, 3 and 4. rlt`hearmature 2l in its repose position lies in a plane parallel to the planeof the magnetic circuit 23 and is displaced therefrom by a fraction lessthan half of its maximum displacement from its repose position duringoscillation. When the armature is displaced in one direction itsmovement is entirely away from the core 23 while during its displacementin the other direction from the repose position it moves into and pastthe position of alignment with the magnetic circuit 23. In thisarrangement also the natural frequency of oscillation of the magneticsystem must be double the mains frequency, that is to say, 100 c./s. foruse with 50 c./s. mains, and the arrangement produces good synchronizingcharacteristics, the natural frequency of oscillation of the magneticsystem being raised when power is applied.

It may be convenient to be able to use arrangements such as those shownin FIGURES 3, 4 and 5 with magnetic systems which have a naturalfrequency of 50 c./s. and this may be achieved by using a rectifier inthe circuit feeding the winding 24 and such an arrangement is shown byway of example in FIGURE 6. As before, the armature 21 oscillates in thegap 22 in the magnetic circuit 23. The winding 24 has a rectifier 25connected across it and an impedance, represented by the resistance 26,is connected in series with the electric mains to limit the currentthrough the rectifier 25. When the mains current flows in one directionthe resistance of the rectifier 25 is very high, so that the normalcurrent flows through the winding 24 but when the main current ows inthe reverse direction the rectifier 25 provides a virtual short circuitof the winding 24, so that practically no current ows through thiswinding.

If the mains supply should fail, winding 24 is deenergized so that theelectromagnet effectively ceases to exist and the natural frequency ofthe oscillating magnetic system returns to its original level, so thatthe magnetic escapement continues to control the clock at its correctspeed.

A substantial increase in oscillation amplitude resulting fromresonance, or an approach to the resonant state, may result inmalfunctioning in several ways. The oscillating system may be pulled outof step-that is, out of synchronism-with the escapement wheel, or it maybe pulled out of phase, although still running in synchronism with thewheel. These faults may result in irregular running. An excessiveoscillation amplitude may also cause the oscillating system to strikestationary parts of the structure, resulting in noisy running or actualdamage. These possibilities are substantially eliminated by theinvention.

According to a further feature of the invention a protective device maybe included in the clock, which comes into operation in case of certaintypes of malfunctioning.

It is know, in mechanisms of this kind, to provide a guard wire. Thisconsists of a short length of springy wire attached to the oscillatingmagnet, a part of the length of the wire lying substantially parallel tothe oscillation axis. The guard wire is so placed that when theoscillating system and the escape wheel are running correctly the guardWire is carried over ythe top of each tooth of the wheel and passes downbetween each adjacent pair of teeth without touching them. If theoscillating magnet should get out of phase or out of step with the wheelthen the guard wire strikes the teeth in turn until correct operation isrestored. In an improved version of this arrangement for use with thisinvention two teeth, or two pairs of teeth, at approximatelydiametrically opposite points on the wheel, are made longer than theother teeth. If the magnetic lock should be broken, or if theoscillating system should begin to oscillate at a harmonic of theintended frequency, the guard wire strikes the long teeth, which causesthe escape wheel to stop and restart from rest. If, however, uponenergizing the electromagnet, the alternating lield is out of phase withthe mechanical oscillation of the magnetic system, the escapement wheelis allowed to rotate through part of a revolution, so that theoscillating system may stabilise itself in phase with the A.C. supply,before the guard pin can operate.

If the escape wheel has an even number of teeth then the long teeth areat diametrically opposite points on the wheel, but in some instances itmay be preferable to have the teeth or pairs of teeth'arranged so thatthey are not quite diametrically opposite, for example, by using an oddnumber of teeth on the wheel.

We claim:

l. A time control mechanism comprising an escapement wheel having a wavymagnetic track therearound and an oscillatory magnetic systemco-operating therewith, an armature consisting of a flat thin strip ofmagnetic material attached to the magnetic system so as to beoscillatory therewith about a repose position, and an electro-magnethaving a winding and a magnetic circuit as a thin strip of magneticmaterial with two ends thereof presenting opposed co-planar pole piecesseparated to provide a gap in the magnetic circuit, the armature beingoscillatory in a path which extends at least into the gap, and the planeof the armature being substantially parallel to the common plane of theco-planar pole pieces when the armature is in the repose position.

2. A mechanism as claimed in claim 1, in which the armature in itsrepose position lies in the common plane of the co-planar pole pieces.

3. A mechanism as claimed in claim l, in which the armature in itsrepose position lies wholly outside the gap.

4. A mechanism as claimed in claim 1, in which the armature in itsrepose position lies partly within and partly outside the gap.

5. A time control mechanism comprising an escapement wheel having a wavymagnetic track therearound and an oscillatory magnetic systemco-operating therewith, an armature consisting of a dat thin strip ofmagnetic material attached to the magnetic system so as to beoscillatory therewith about a repose position, and an electro-magnethaving a winding, a half-wave rectifier connected with the winding, anda magnetic circuit as a thin strip of magnetic material with two endsthereof presenting opposed co-planar pole pieces separated to provide agap in the magnetic circuit, the armature being oscillatory in a pathwhich extends at least into the gap, and the plane of the armature beingsubstantially parallel to the common plane of the co-planar pole pieceswhen the armature is in the repose position.

References Cited in the file of this patent UNITED STATES PATENTS1,989,604 Poole Ian. 29, 1935 FOREIGN PATENTS 698,406 Great Britain Oct.14, 1953

1. A TIME CONTROL MECHANISM COMPRISING AN ESCAPEMENT WHEEL HAVING A WAVYMAGNETIC TRACK THEREAROUND AND AN OSCILLATORY MAGNETIC SYSTEMCO-OPERATING THEREWITH, AN ARMATURE CONSISTING OF A FLAT THIN STRIP OFMAGNETIC MATERIAL ATTACHED TO THE MAGNETIC SYSTEM SO AS TO BEOSCILLATORY THEREWITH ABOUT A REPOSE POSITION, AND AN ELECTRO-MAGNETHAVING A WINDING AND A MAGNETIC CIRCUIT AS A THIN STRIP OF MAGNETICMATERIAL WITH TWO ENDS THEREOF PRESENTING OPPOSED CO-PLANAR POLE PIECESSEPARATED TO PROVIDE A GAP IN THE MAGNETIC CIRCUIT, THE ARMATURE BEINGOSCILLATORY IN A PATH WHICH EXTENDS AT LEAST INTO THE GAP, AND THE PLANEOF THE ARMATURE BEING SUBSTANTIALLY PARALLEL TO THE COMMON PLANE OF THECO-PLANAR POLE PIECES WHEN THE ARMATURE IS IN THE REPOSE POSITION.